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445e1bb9cf
commit 3220022038 upstream.
clang-15's ability to elide loops completely became more aggressive when
it can deduce how a variable is being updated in a loop. Counting down
one variable by an increment of another can be replaced by a modulo
operation.
For 64b variables on 32b ARM EABI targets, this can result in the
compiler generating calls to __aeabi_uldivmod, which it does for a do
while loop in float64_rem().
For the kernel, we'd generally prefer that developers not open code 64b
division via binary / operators and instead use the more explicit
helpers from div64.h. On arm-linux-gnuabi targets, failure to do so can
result in linkage failures due to undefined references to
__aeabi_uldivmod().
While developers can avoid open coding divisions on 64b variables, the
compiler doesn't know that the Linux kernel has a partial implementation
of a compiler runtime (--rtlib) to enforce this convention.
It's also undecidable for the compiler whether the code in question
would be faster to execute the loop vs elide it and do the 64b division.
While I actively avoid using the internal -mllvm command line flags, I
think we get better code than using barrier() here, which will force
reloads+spills in the loop for all toolchains.
Link: https://github.com/ClangBuiltLinux/linux/issues/1666
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
19 lines
521 B
Makefile
19 lines
521 B
Makefile
# SPDX-License-Identifier: GPL-2.0
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#
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# Copyright (C) 1998, 1999, 2001 Philip Blundell
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#
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obj-$(CONFIG_FPE_NWFPE) += nwfpe.o
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nwfpe-y += fpa11.o fpa11_cpdo.o fpa11_cpdt.o \
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fpa11_cprt.o fpmodule.o fpopcode.o \
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softfloat.o single_cpdo.o double_cpdo.o \
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entry.o
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nwfpe-$(CONFIG_FPE_NWFPE_XP) += extended_cpdo.o
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# Try really hard to avoid generating calls to __aeabi_uldivmod() from
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# float64_rem() due to loop elision.
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ifdef CONFIG_CC_IS_CLANG
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CFLAGS_softfloat.o += -mllvm -replexitval=never
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endif
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